mempool: get capabilities

Allow the mempool driver to advertise his pool capabilities.
For that pupose, an api(rte_mempool_ops_get_capabilities)
and ->get_capabilities() handler has been introduced.
- Upon ->get_capabilities() call, mempool driver will advertise
his capabilities to mempool flags param.

Signed-off-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>